There has recently been a rapid advancement in realization of small, lightweight and cordless electronic appliances such as personal computers and portable telephones, leading to a high demand for secondary batteries having a high energy density as power sources for these appliances. Among them, a non-aqueous electrolyte secondary battery containing lithium as an active material has been expected greatly to be a battery having a high voltage and high energy density. This battery conventionally comprised a negative electrode using metal lithium and a positive electrode using molybdenum disulfide, manganese dioxide, vanadium pentoxide or the like, to be a 3V-level battery.
In the case that metal lithium is used for the negative electrode, however, there is a problem that dendritic lithium deposits on the electrode plate during charging and is accumulated thereon as the charging and discharging are repeated, the accumulated dendritic lithium being isolated, floating in the electrolyte and coming into contact with the positive electrode to incur a minor short. This results in a short cycle life as charge and discharge efficiency of the battery falls. And there is another problem in terms of safety since dendritic lithium has a large surface area and thus has the high reaction activity.
In order to solve these problems, vigorous studies have been done in recent years on a lithium-ion secondary battery in which a carbon material is used for the negative electrode in place of metal lithium, and a lithium-containing transition metal oxide having a 4V-level potential against lithium such as LiCoO2, LiNiO2 or LiMn2O4, is used for the positive electrode, the battery having already been commercialized. In this battery, lithium is in the state of being absorbed as ions in the carbon material in the negative electrode. This prevents the deposition of dendritic lithium, which was observed on the conventional negative electrode using metal lithium, and thus enables the battery to ensure extremely high reliability in safety.
As thus described, the characteristics of the positive electrode and the negative electrode are of importance in a non-aqueous electrolyte secondary battery, particularly in a lithium-ion secondary battery. And further, if satisfactory characteristics are to be obtained, the characteristic of a non-aqueous electrolyte transferring lithium ions is also of importance. A non-aqueous solvent composing this non-aqueous electrolyte is usually a combination of a solvent having a large dielectric constant which facilitates dissolution of a solute and a solvent having low viscosity.
The reason for the combination is as follows:
The solvent having a large dielectric constant has high viscosity and thus transfers ions very slowly. Then a solvent having low viscosity is also used so-as to enhance capability of transferring ions. For example, a cyclic carbonic acid ester, which is the solvent having a large dielectric constant, such as ethylene carbonate, and a linear carbonic acid ester, which is the solvent having low viscosity, such as dimethyl carbonate, diethyl carbonate or ethylmethyl carbonate are mixed to obtain an electrolyte having high conductivity, which has hitherto been in general use. Because ethylene carbonate has a solidifying point of as high as around 38° C., when it is singly used, the solidifying point may go down to around 0° C. at the lowest even with the consideration of the freezing point depression due to the solute dissolved therein. As described above, therefore, ethylene carbonate is mixed with the solvent having low viscosity as well as a low solidifying point to ensure low temperature characteristics. Under the present circumstances, however, even the mixed solvent cannot ensure sufficient low temperature characteristics since no small effect of ethylene carbonate on the low temperature characteristics remains.
Thereupon, an electrolyte using a lactone-type solvent, which is a cyclic carboxylic acid ester, has been proposed (in Japanese Laid-Open Patent Publication No. Hei 11-097062). This is a very preferable solvent for the lithium-ion secondary battery as having a solidifying point of as low as −45° C. while having a large dielectric constant.
However, γ-butyrolactone, one of lactone-type solvents, has a drawback of being prone to reductively decomposed on the negative electrode, which leads to generation of a large amount of decomposition gas in the battery. In order to suppress the reductive decomposition of γ-butyrolactone on the negative electrode, a battery has been studied in which vinylene carbonate, known as an additive to form a film on a negative electrode, is added to an electrolyte containing γ-butyrolactone. But this battery exhibits significant deterioration in charge and discharge characteristics when exposed to a high temperature over a period of time. This is presumably because the decomposition of vinylene carbonate on the negative electrode is accelerated by heat to form an excessive film on the negative electrode. That is, the excessive film inhibits the smooth intercalation and deintercalation of lithium ions into and from the negative electrode, resulting in significant deterioration of charge and discharge characteristics of the battery after exposure of the battery to a high temperature over a period of time.